SOHO Antenna Problem

The Solar and Heliospheric Observatory (SOHO) spacecraft expects to experience a blackout in the transmission of its scientific data during the week of 22 June 2003. This is estimated to last for about two and a half to three weeks.

Engineers are predicting this problem after detecting a malfunction in the pointing mechanism of the satellite's high-gain antenna (HGA), which is used to transmit the large amounts of data from SOHO's scientific observations to Earth.

The SOHO spacecraft is operating as safely as before the problem occurred. Its low gain antenna, which does not need to be pointed in a specific direction (omni-directional), will be used to control the spacecraft and monitor both spacecraft and instrument health and safety.

The anomaly in pointing the high-gain antenna was recently discovered when engineers detected a discrepancy between the commanded and measured antenna position. In normal conditions, the antenna must be able to move along two axes, vertical and horizontal. The horizontal movement was no longer taking place properly. The problem is probably due to a malfunction in the motor or gear assembly that steers the antenna.

SOHO is located 1.5 million kilometers from Earth, slowly orbiting around the First Lagrangian point, where the combined gravity of the Earth and the Sun keep SOHO in an orbit locked to the Sun-Earth line. To transmit data, the SOHO high-gain antenna must rotate to have the Earth constantly in its field of view as the spacecraft and the Earth progress in their respective orbits.

If the problem is not solved, the Earth will be left outside the HGA beam on a periodic basis, with similar blackouts occurring every three months.

ESA and NASA engineers are currently assessing several options to recover the situation, or minimise the scientific data loss.

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